Panels of gypsum wallboard having a core of set gypsum sandwiched between two sheets of facing paper have long been used as structural members in the fabrication of buildings. Such panels are typically used to form the partitions or walls of rooms, elevator shafts, stairwells, ceilings and the like. Paper facing provides a smooth surface that is especially desirable for painting or wall papering interior walls. Although paper is a relatively inexpensive facing material and is easily used in the process of manufacturing wallboard, it has certain disadvantages, particularly with regard to durability and moisture-resistance.
As an alternative to paper facing, other fibrous mats (such as glass fiber mats) also have been used as facing materials for gypsum wallboard. One example of such a wallboard is described in U.S. Pat. No. 3,993,822. Fibrous glass matting provides improved water resistance and often provides significant improvements in strength and other structural attributes. More recently, fibrous glass mats having various types of coatings also have found acceptance for use in applications requiring moisture resistance.
One specialty application for the use of panels of gypsum wallboard of this construction is in bathrooms—typically a place of high humidity and residual water because of the flow of water from the use of showers, bathtubs, and sinks. Gypsum wallboards suitable for use in these applications share a common requirement; that is a resistance or tolerance to high humidity and high moisture environments, often for prolonged periods.
A usual construction of bathroom walls includes a multi-ply structure of ceramic tile adhered to an underlying base member, for example, a panel of wallboard comprising gypsum or other material. Such a panel is referred to in the industry as a “tile backing board,” which for convenience is referred to herein as “tile backer”. In usual fashion, sheets of tile backer (for example, 4′×8′×½″) are fastened by rust-resistant nails or screws to studs. Blocks of ceramic tiles (for example, 4″×4″) are adhered to the sheets of tile backer using a water-resistant adhesive which is referred to in the industry as “mastic” or by a Portland cement-based adhesive which is referred to commonly as “thin set mortar”. Thereafter, spaces between the tiles and between the tiles and other adjoining surfaces, for example, the lip of a bathtub or sink, are filled with a water-resistant material which is referred to in the industry as “grouting”.
It should be appreciated that a primary goal in constructing a bathroom that includes one or more of a bathtub, shower and sink is to make the contiguous and adjacent walls water-tight utilizing materials that resist being degraded by water, including hot water. Tiles made from ceramics are such materials and are basically inert to both the hot and cold water with which the tiles come into direct contact.
It is important also that the tile backer to which the tiles are adhered be water-resistant. Theoretically, it would seem that the water-resistant properties of the tile backer should be inconsequential because the backer is shielded from shower, bath and sink water by water-resistant tiles, grouting and mastic. However, experience has shown this is not the case and that moisture can and does in fact seep, in various ways, through the plies of material which overlie the tile backer.
One way has to do with the fact that grouting is not water-impervious and over time permits the seepage of moisture, a situation which is aggravated upon the formation of cracks, including hairline cracks, in the grouting. Eventually, the moisture which penetrates through the grouting finds its way through the mastic and comes into contact with the facing of the wallboard. Such facing is generally paper, typically a multi-ply paper, which upon contact with moisture tends to degrade by delaminating or otherwise deteriorating. For example, the paper facing may be subject to biological degradation from mold and mildew. The paper can actually rot away. Furthermore, as the moisture comes into contact with the underlying set gypsum core, it tends to dissolve the set gypsum and also the core adhesive, which bonds the core and paper facing together. Such adhesive is typically a starch material. The development of these conditions can lead to tiles coming loose from the underlying deteriorated paper-faced gypsum wallboard. This undesirable situation is exacerbated when hot water comes into contact with the paper-faced wallboard.
Another type of moisture condition which leads to the loosening or falling off of tiles from their underlying support substrate is associated with those segments of the multi-ply wall structure which include a joint formed from an edge portion of the wallboard. An example is the joint formed by the edge of a wallboard panel and the lip of a bathtub. Another example is the joint formed by two contiguous wallboard panels. As moisture penetrates through the multi-ply structure and reaches such a joint, it tends to wet significant portions of the paper facing and core by virtue of its spreading through capillary action. This can lead to delamination of the paper facing and/or dissolution of the core and/or the paper/core adhesive. As this occurs, tiles can come loose and fall off.
One water-resistant gypsum panel suitable for use in such moisture-prone conditions is described in U.S. Pat. Nos. 5,397,631 and 5,552,187. According to these patents, following the manufacture of a fibrous glass mat-faced gypsum panel, a surface of the panel faced with a glass mat is coated with a substantially humidity- and water-resistant resinous coating of a cured (dried) latex polymer. The coating acts as both a liquid and vapor barrier and is formed from an aqueous coating composition comprising from about 15 to about 35 wt. % of resin solids, about 20 to about 65 wt. % of filler, and about 15 to about 45 wt. % of water, applied to obtain a solids loading of about 110 lbs. per 1000 sq. ft. A preferred resin for use according to this patent is a latex polymer that has been sold by Unocal Chemicals Division of Unocal Corporation under the mark 76 RES 1018. The resin is a styrene-acrylic copolymer that has a relatively low film-forming temperature. Aqueous coatings formed from the resin are dried effectively at temperatures within the range of about 300° to 400° F. If desired, a coalescing agent can be used to lower the film-forming temperature of the resin.
While this approach produces a gypsum panel that satisfactorily solves many of the previous-mentioned problems encountered when using paper-faced gypsum panels in severe moisture environments, the added cost, due both to the cost of the resinous coating itself and the cost associated with how the coating is applied, has been an impediment to wider use of such panels.
One important embodiment of the present invention thus relates to an improved gypsum-based structural panel having a water impervious coating, such that the panel can be used effectively as a tile backer. Still other embodiments of the improved gypsum panel may have use in other applications such as in the return air installations, shaft walls and area separator walls in commercial buildings where water and humid conditions are commonly encountered. Other applications where moisture and humid conditions are likely to present difficulties either during the installation or the use of the board also will be apparent to those skilled in the art.
These and other embodiments of the invention, which relies on the provision of a radiation cured, e.g., ultraviolet (UV) cured, coating on a fibrous mat faced gypsum panel, will be apparent from the following description.